If the thermostat terminals read 24V but the gas valve coil reads 0V in a no-heat call, which component is likely open?

In a heating system, when the thermostat is set to call for heat, it typically sends a signal by closing a circuit, which should result in a voltage reading at the gas valve coil. If the thermostat terminals are reading 24V but the gas valve coil is reading 0V during a no-heat call, this indicates that the signal from the thermostat is not reaching the gas valve.

The thermostat acts as the control center in the system, and if it is functioning correctly and showing a voltage at its terminals, it should be able to effectively transmit that voltage to the gas valve coil. Since the gas valve coil is reading 0V, it suggests that the connection between the thermostat and the gas valve coil is interrupted, likely due to an issue within the thermostat itself.

This could mean that the thermostat is either stuck in the "off" position, malfunctioning, or not properly closing the circuit to allow the current to reach the gas valve. Therefore, the thermostat is the component that is likely open, preventing the current necessary to operate the gas valve coil. Understanding this helps in troubleshooting the system effectively, as further checks would need to be performed on the thermostat to ensure it is functioning properly.